Usually an LED array head includes several thousand LED's and in order to drive these LED's several tens to several thousand integrated circuits each having 32 to 64 driver circuits must be used.
Conventionally, as LED driving integrated circuits, MOS integrated circuits shown, for example, in the official gazette of Unexamined Japanese Patent Publication No. SHO 60-198872 have been used.
FIG. 1 shows an LED driving MOS integrated circuit disclosed in the above-stated Unexamined Japanese Patent Publication No. SHO 60-198872.
Referring to FIG. 1, an LED driving circuit 1 controls power supplied to LED's 3a-3c from a DC power supply 2 so that the brightness of the LED's is maintained at a predetermined value. The LED driving circuit 1 is fabricated as an MOS integrated circuit on a single substrate.
The LED driving MOS integrated circuit 1 comprises MOS transistors 4 connected in series with the respective LED's 3a-3c The MOS transistors 4 serve as current limiting elements for limiting the values of currents flowing through the respective LED's 3a-3c. The MOS transistors 4 are connected to the positive terminal of the DC supply 2 through associated resistors 5 A desired LED driving current limited by the MOS transistors 4 flows through the associated resistors 5, and a voltage V.sub.1 at a level determined by the magnitude of current flowing through the respective ones of the LED's 31-3c is developed across the resistor 5. The voltage V.sub.1 is applied to a .varies. input terminal of an associated voltage comparator 6.
A depletion-type MOS transistor 7 having its gate electrode connected directly to its own source electrode is used to derive a reference voltage V.sub.r which is at a constant level regardless of the level of the terminal voltage V.sub.O of the power supply 2 and is applied to -input terminals of the respective voltage comparators 6 The drain electrode of the transistor 7 is connected to the -input terminals of the respective voltage comparators 6 and also connected through a resistor 8 to the positive terminal of the power supply 2. The source electrode of the transistor 7 is grounded together with its gate electrode
The drain current I.sub.D of the transistor 7 flowing through the resistor 8 causes the voltage V.sub.r to be developed across the resistor 8, and this voltage is applied as a reference voltage to the -input terminals of the voltage comparators 6.
Thus each of the voltage comparators 6 develops an output voltage V.sub.G in accordance with the difference between the voltage V.sub.1 and the reference voltage V.sub.r. This output voltage V.sub.G controls the conduction of the associated transistor 4. When V.sub.l &lt;V.sub.r, the output voltage V.sub.G of each voltage comparator 6 increases in the positive direction to increase the drain current of the transistor 4 which in turn causes the voltage drop across the associated resistor 5 to increase. When V.sub.l &gt;V.sub.r. the output voltage V.sub.G of each voltage comparator 6 increases in the negative direction so that the drain current of the transistor 4 decreases, which in turn causes a decrease in the voltage drop across the resistor 5. Thus, the voltage V.sub.1 decreases. In this way, the conductions of the transistors 4 are Controlled so as to produce the V.sub.l =V.sub.r condition, and, accordingly, current in accordance with the level of the reference voltage V.sub.r flows through the respective transistors 4 into the LED's 3a-3c . Thus, the LED's 3a-3c are driven with constant current.
Switching transistors 9 are connected between the gates of the respective MOS transistors 4 and ground, respectively, for selectively driving the LED's 3a, 3b and 3c. The switching transistors 9 are turned on or off in response to a selection signal applied thereto from a control circuit 10, so that the desired LED's are selectively energized to emit light.
In the above-stated conventional type of LED driving integrated circuits, the resistors 5 and 8 are usually formed simultaneously as diffused resistors or polysilicon resistors, and, therefore, there is little variation in resistance value of he resistors in a particular one of the integrated circuits. However, depending on manufacturing conditions, variation of more than .div.35% could be caused in resistance value among different integrated circuits. Accordingly, variations more than .+-.35% in output current from such integrated circuits will be caused. That is, the output current of one integrated circuit could be two or more times that of other integrated circuits.
The quality of pictures produced by a printer depends on the uniformity of the light output of its LED heads. Therefore it is necessary to make the light output of the LED head as uniform as possible so that variations in light output may be, for example, within a range of less than .+-.20%. Although not discussed herein, there are other major causes for variations of the light outputs and variations of integrated circuits, namely, variations of LED array chips and focusing lenses. Therefore, in order to provide uniform light outputs at least variations of driving power supplies of the integrated circuits must be reduced to, for example, less than .+-.5%. In order to realize it, it is necessary to measure the output currents of respective integrated circuits and select those integrated circuits which have like values of the output currents. However, such a classification procedure is one of the largest causes of increase of the manufacturing costs.
The light output of an LED is temperature-dependent, and as temperature increases. The light output of an LED decreases. Accordingly, it is necessary that current to drive the LED be controlled in response to temperature. In the above-described conventional MOS integrated circuit 1 for driving LED's, the depletion-type MOS transistor 7 is used to develop the reference voltage V.sub.r for temperature-controlling the drive current. However, the value of V.sub.r varies from circuit to circuit due to varying manufacturing conditions which cannot be made constant, so the yield of usable integrated circuits is low, which raises the cost of the integrated circuits.
The present invention is to eliminate the above-stated disadvantages of conventional integrated circuits. According to the present invention, variations in values of output currents of integrated circuits can be minimized, so that no procedure for classifying manufactured integrated circuits is needed and the yield of usable integrated circuits is increased. Thus, the manufacturing costs of LED array heads and, hence, ultimate products Can be reduced.